The present invention relates to a three-beam-type optical pickup and a hologram which is suitable for use in this type of optical pickup.
FIG. 1 shows a constitution of a conventional three-beam-type optical pickup. In this figure, a laser light beam emitted from a laser diode 1 is split into three beams through diffraction by a diffraction grating 2, and split light beams pass through a non-polarizing beam splitter 3 and a collimator lens 4. Light beams which were made into parallel beams by the collimator lens 4, are focused by an object lens 5 so as to form light spots on a signal surface of a disk 6. Return light beams reflected by the disk signal surface return to the beam splitter 3 through the same light paths as their incident paths, are reflected by the beam splitter 3 to the direction of a detector 9, and pass through a plano-concave lens 7 and a cylindrical lens 8. The return light beams transmitted from the cylindrical lens 8 form spots on a 6-piece-divided photodetector 9. Shapes of the spots on the 6-piece-divided photodetector 9 are also illustrated in FIG. 1 for three cases (a), (b) and (c) in which the disk signal surface is just in the focusing plane, the optical pickup is too near to the signal surface, and the optical pickup is too far from the signal surface, respectively.
In this case, a focusing error signal (F.E.), a tracking error signal (T.E.), and a reproduced RF signal (R.F.) are expressed as the following respective equations:
F.E.=( A+C )-( B+D ) PA1 T.E.=E-F PA1 R.F.=A+B+C+D
where A to F represent outputs from respective photosensors of the 6-piece-divided photodetector 9. Detecting methods of the above focusing error and the tracking error include an astigmatism method and a three beams method, respectively.
The foregoing conventional three-beam-type optical pickup necessitate eight devices: the laser diode 1, the diffraction grating 2, the beam splitter 3, the collimator lens 4, the object lens 5, the plano-concave lens 7, the cylindrical lens 8, and the 6-piece-divided photodetector 9. As these many optical devices are necessary, the conventional three-beam-type optical pickup is costly, it is not easy for the pickup to be miniaturized and made light, and it is complicated to adjust the pickup because of many adjusting points.
Moreover, though there exists a hologram having two grating areas which are constituted in such a manner that diffraction light beams from the hologram are focused on different two points, the two grating areas are formed on one surface of the hologram.